Heating of a packaging pouch hermetically filled with a retort food, a frozen food, or the like in a microwave oven increases the pressure inside the pouch due to water vapor or the like being generated from its content under heating. The packaging pouch may tear, and the content may scatter, to thereby make the inside of the microwave oven dirty or harm a human body by burning or the like.
Thus, before heat-cooking such a packaging pouch in a microwave oven, a method of preventing tearing of the packaging pouch is employed. That is, the packaging pouch is partly opened in advance or the packaging pouch itself is pierced, to thereby discharge water vapor or the like that is generated inside the packaging pouch.
However, such a method involves much time and effort for a general consumer. Further, this method has a disadvantage in that flavor is lost because the water vapor generated by microwave heating is immediately discharged out of the packaging pouch and heat steaming effect of the water vapor on the content is reduced.
There are known various packaging pouches each provided with means for automatically relieving an increase in the inner pressure of the packaging pouch under heating in a microwave oven. Proposed examples of a conventional technique include: (1) a packaging pouch provided with a butt-seam part that is seamed to the packaging pouch through a heat seal of a predetermined width, in which the butt-seam part is formed of an easily opened seal (JP-A-2001-106270, for example); (2) a packaging pouch provided with a weak seal part partly formed in a heat seal part of the packaging pouch by interposing a thin film, and partly provided with a narrow seal part of a small heat seal width in the weak seal part (JP-A-10-59433); and (3) a packaging pouch provided with a branched section (fin part), and provided with automatic opening means for inner vapor in the branched section (see JP-A-2002-80072, JP-A-2003-2374, and JP-A-2003-81360).
However, the packaging pouch provided with automatic opening means for inner vapor in the branched section as described in the conventional technique (3) has a problem in that a heat seal part of the packaging pouch peels off from a base part of the branched section during retort treatment under conditions of 120° C. for 30 minutes or during heat-cooking of the packaging pouch filled with a contents with a high power microwave oven, such that the packaging pouch is liable to open to leak the contents or is liable to tear.
Thus, an object of the present invention is to provide at a low cost a packaging pouch for a microwave oven having a branched section capable of preventing opening of the packaging pouch from a base part of the branched section during retort treatment or during heat-cooking in a microwave oven and capable of automatically releasing pressure inside the packaging pouch during heat-cooking in a microwave oven.